It can be difficult to obtain back fat for making your own sausage, I tend to use the rind from skins that Morrison's sell for making crackling, this usually yields a reasonable amount, recently Morrison's had pork poin on offer at 4.79 a kilo, so I bought a full loin just under 4 kg, only to be horrified that the butcher started to trim of the fat, so I asked him to bag it seperately, lovely white firm fat, ideal for sausage, he told me they trimmed all the loins, I couldn't believe it when he showed a huge bag of white fat that was going in the food skip, so I asked if they could keep me some to one side and I would gladly make a contribution into one of their charity boxes, magic, I now have all the fat I need for making my sausage and black pudding.

Same here in Greece, Greeks don't want fat & won't pay for it & so all the fat is trimmed from Pork loin, shoulder etc & is just thrown away! I buy half or whole pigs these days to cut as I want, all back fat that I need is free, as are pigs kidneys, which are also just discarded.

Here on the east coast of the US, supermarkets have largely gone to central meat processing, so the meat that is delivered to the store is already trimmed to death and packaged, for the most part. The "specialty cut" counter as "meat cutters" who use a steaking knife to cut strip steaks, ribeyes, and pork chops, but they are pre-trimmed, as well. Even some of the "butcher shops" in the area get their meat as pre-trimmed primals. Very sad. My source for local back fat went out of business, so I'm back to ordering it by the case at about $3.50/lb.

If you can't get back fat, or order it from a meat processor, you should be able to get 70/30 or 80/20 trim from a meat processor.